Wednesday, November 21, 2012

Weather Whys Wednesday: Thundersnow

Now that we are beginning to creep into winter, obviously the focus of interest and conversation is going to change. Over the next few weeks, we'll tackle some winter weather topics you may have some interest in. Let's discuss thundersnow first.

Thundersnow is probably the most "weenie-riffic" winter weather element. It truly is an exciting concept and combines all the mysterious elements that seem to excite weather enthusiasts. I think it's partially because most weather enthusiasts are snow obsessed, but also because it's an infrequent event, but not so much that it could be deemed as "rare." It was perhaps made most famous by The Weather Channel's Jim Cantore, who seems to always be on the receiving end of thundersnow when he covers winter storms.

So let's learn more about it. The biggest characteristic that differentiates thundersnow from your thunderstorm is really just the timing, location, and type of storm it occurs in. Usually, the lightning we see in summer/spring storms is associated with tall, billowing cumulonimbus clouds, ahead of and along cold fronts, etc. In winter, thundersnow usually occurs in rather low topped convection, forced most often by either strong vertical motion at the synoptic level (in the case of a blizzard or nor'easter, such as the Blizzard of 1996 shown in the above video) or forced at a more mesoscale level in convective lake effect snow bands (which we'll touch on in a future entry).

It can also occur with a cold frontal passage as well, but more often than not, the differences in air mass from before the front to behind it would have to be fairly extreme or very cold (most often in a secondary push of Arctic or Polar air, as opposed to a generic cold front). Those are most often the 10-20 minute bursts of tremendously heavy snow and whiteout conditions that can occasionally cause massive traffic disruptions.

Now, within the larger scale storms, thundersnow most often forms in the comma head of the cyclone. This is usually on the northwestern side of the storm, which helps the Delaware Valley occasionally see it during offshore tracking nor'easters. You can also occasionally see thundersnow ahead of (north and west of) an approaching cyclone. It normally would occur with an inverted trough that may occasionally form extending out to the north of the main low into the colder air. This is called a trough of warm air aloft (or TROWAL).

In lake effect precipitation, where thundersnow is actually somewhat common, you need to have a certain temperature differential between the lake water and the air at about 5,000 feet. In the upcoming LES entry, we'll discuss this some more.

What thundersnow does usually promise you is heavy snow. Rates usually get to at least 2-3" per hour in thundersnow, and I've seen lake effect bands produce thunder and as much as 5-7" per hour. Think of some of the more absurd rain scenarios you can have with thunderstorms and translate them to winter and you can see just how heavy it can get. Say what you will about winter, snow, cold, etc., but thundersnow to this meteorologist still remains one of the neatest weather elements that exists.